FAQ - LDPE-LLPDE-HDPE

Polyethylene is a polyolefin which is an high molecular weight hydrocarbon. These are the only plastics that have a lower specific gravity than water. The main properties of polyethylene are to be weatherproof, easy to process, low cost and to warranty a good chemical resistance.
When ethylene is polymerized the result is relatively straight polymer chains. From the main chain they can branch out. We get different kinds of Polyethylenes from the varying degree of branching in their molecular structure.

– LDPE (Low Density Polyethylene) has the most excessive branching. This causes the low density to have a less compact molecular structure which is what makes it less dense. It has a density of 0.910-0.925 g/cm3. LLDPE (Linear Low Density Polyethylene) has a significant numbers of short branches. Because it has shorter and more branches its’ chains are able slide against each other upon elongation without becoming entangled like LPDE which has long branching chains that would get caught on each other. This gives LLDPE higher tensile strength and higher impact and puncture resistance than the LDPE. It has a density of 0.91-0.94 g/cm3.
Low Film for packaging is one of the largest markets for low density polyethylene (LDPE/LLDPE). Others applications are : squeeze bottles, toys, carrier bags, high frequency insulation, chemical tank linings, heavy duty sacks, general packaging, gas and water pipes.

– HDPE (High Density Polyethylene) has minimal branching of its’ polymer chains. Because it is denser it is more rigid and less permeable then the LDPE. It has a density of 0.941-0.965 g/cm3. Applications of HDPE are : chemical drums, jerricans, carboys, toys, picnic ware, household and kitchenware, cable insulation, carrier bags, food wrapping material. Low cost, high rigidity and ease of blow moulding has made the material a natural choice in gardening furnishings.

– Some others polyethylene could be find and are commonly used : MDPE (Medium Density Polyethylene), UHMWPE (Ultra High Molecular Weight Polyethylene) and XLPE (Crosslinked Polyethylene).

The LDPE process consists in five operations :
– The compression of gas : Gaseous ethylene is supplied and melted with a part of unreacted gas from the process in oreder to be compress in the first reactor.This new compressed gas is melted again with unreacted gas and compress in the second compressor.
– The polymerization : An initiator (organic peroxide) is added to the second compressed gas into the reactor and the materials are mixed inside the reactor through stirrer . Polymerization is obtained in the reactor at a certain pressure and temperature.
– The separation of gas : The unreacted gas is then separated by 3 levels of separators. Those unreacted gas will be injected before the compressor, notice than a part will be exclude from the process.
– The extrusion : Once the unreacted gas is removed, the polymers can be extruded and pelletized.
– The storage and packaging : The pellets are dried through a dryer and classified pellets by pellet size. The degassing is done by hot air injection.

The HDPE process consists of :
– Polymerization : Ethylene monomers are polymerized in solvent together with catalyst, hydrogen and comonomer. The polymerization heat is cooled through external circulation heat exchanger. The reacted slurry is transferred to the separation/drying process.
– Separation/Drying : Slurry is transferred to a high-speed centrifuge from which it is separated into solvent and wet powders. The separated solvent is supplied to the reactor and some solvents are recycled in the process through refining. Wet powders are transferred to the powder dryer and dried.
– Transfer/Extrusion : The wet powders are dried in the powder dryer by evaporating the solvent with high-temperature nitrogen and steam. The evaporated solvent is recovered by the scrubber. The dried powders are transferred to the extrusion process where they are melted and pelletized in the extruder. Then they are transferred to the storage silo.
– Storage & Packaging : The products transferred to the pellet silo are cooled by air and homogenized.

Guichon Valves offers valves for polyethylene production specially designed for viscous liquids and designed to minimize pressure drops and dead zones. Jacketed valves allow to maintain the fluid temperature to prevent solidification of polyethylene. Sealants (gaskets, seals, etc …) are to Selected to prevent pollution of polyethylene.
Guichon Valves offers specially designed valves suited for abrasive application like powder and pellets.

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Guichon Valves,manufacturer of valvessince 1921

For more than 90 years, Guichon Valves, an independent european company, led by a long-term oriented management team, is focused on design, manufacturing, assembling and testing of special and custom-made valves, for petrochemical, chemical, fine chemical, pharmaceutical, and nuclear industries, with demanding requirements.